Since the game will be released on both Steam and GOG: Will it be possible to play together with players on other platforms?

A very important question. Hope it will be possible.

I've done the same question on the official JA3 forums, but no one still knows for sure.

> Connection occurs through dedicated servers so the host doesn't have to deal with any port forwarding or network setup. They can just click host and invite their friend without having to worry about whether their friend can connect.

> We have nothing to announce regarding crossplatform play at present, but it is definitely on our radar.

Source

The current version requires Galaxy, so I'd say it's a "no" at the moment.
Maybe a feauture version will use EGS?

I take it that also means no LAN support? Ugh. That would have been the only way I'd ever expect to play this in multiplayer.